1. Field of the Invention
The present invention relates to methods for preparing sodium salts of imidazoledicarboxylic acid derivatives of substituted cephalosporin.
2. Description of the Prior Art
In the specification of laid-open Japanese Patent Application No. 55-76887, salts of imidazoledicarboxylic acid derivatives represented by the following general formula: ##STR3## are disclosed which demonstrate antibacterial properties, particularly against Pseudomonas aeruginosa, and are useful as antibiotics. The synthesis of the salts involves reacting compounds of the following formula ##STR4## with 4-pyridineethanesulfonic acid in the presence of potassium iodide or potassium thiocyanate in order to introduce pyridinium groups. This reaction results in the formation of the potassium salt of compound (I). Refer to The Journal of Antibiotics, 29, 928 (1976).
It is also disclosed in laid open Japanese Application No. 55-76887, mentioned above, that the sodium salts of the imidazolidicarboxylic acid derivatives (I) are obtained by treating their potassium salt counterparts with ion exchange resin and then neutralizing with aqueous sodium hydroxide solution. This step is both complicated and results in a low yield of final product. Additionally since large amounts of potassium iodide or potassium thiocyanate are used in the reaction, separation of the unreacted iodide salt from the imidazoledicarboxylic acid salt is both difficult and expensive, requiring large quantities of synthesized absorbent such as "Amberlite XAD-2", available from Rohm, and Haas Company.
Thus a need has continued to exist for an improved method of preparing the sodium salts of imidazoledicarboxylic acid derivatives of substituted cephalosporins.